Personal “in-ear” monitoring systems are utilized by musicians, recording studio engineers, and live sound engineers to monitor performances on stage and in the recording studio. In-ear systems deliver a music mix directly to the musician's or engineer's ears without competing with other stage or studio sounds. These systems provide the musician or engineer with increased control over the balance and volume of instruments and tracks, and serve to protect the musician's or engineer's hearing through better sound quality at a lower volume setting. In-ear monitoring systems offer an improved alternative to conventional floor wedges or speakers, and in turn, have significantly changed the way musicians and sound engineers work on stage and in the studio.
Moreover, many consumers desire high quality audio sound, whether they are listening to music, DVD soundtracks, podcasts, or mobile telephone conversations. Users may desire small earphones that effectively block background ambient sounds from the user's outside environment.
Hearing aids, in-ear systems, and consumer listening devices typically utilize earphones that are engaged at least partially inside of the ear of the listener. Typical earphones have one or more drivers of either dynamic moving-coil or balanced armature design that are mounted within a housing. Typically, sound is conveyed from the output port of the driver(s) into the user's ear canal through a cylindrical sound port or a nozzle.
Multiple driver earphones can produce a more accurate frequency response especially in the lower frequency range typical of a bass guitar or bass drum. A better quality sound output is realized by optimizing the particular driver for a specific sound region because the particular driver can be designed specifically for a particular frequency range. Additionally multi-driver earphones are able to provide greater volume sound without as much distortion, thereby yielding a cleaner sound in higher decibel settings. However, it is also desired to filter the higher frequencies produced by the low frequency driver to optimize the performance or sound quality of the earphone, as discussed in more detail below.
In a related field, passive electrical methods acting as low pass filters are common in loudspeakers. Loudspeaker cross-over designs often use a simple first order passive electrical network to create low and high pass filters, primarily to allow each speaker to work in its efficient range and to avoid damage to drivers not designed to reproduce particular frequencies. Properly designed crossovers also minimize destructive phase interactions between multiple acoustic sources that reproduce overlapping frequency regions. Appropriately paired low and high pass filters also prevent a parallel electrical network of drivers from presenting an excessively low load impedance to the source amplifier. Passive networks often use inductors to create low-pass filters electronically, with the performance of the inductor directly related to its number of coil turns.
However, in regard to multi-driver earphone design, the use of inductors for low pass filtering presents two significant hurdles in practical implementations. First, the requirement for a large number of turns results in a rather large package size. Second, the use of small gauge wire utilized to maximize the number of turns per unit of inductor volume results in significantly higher values of DC resistance. When placed in electrical series with the receiver, this DC resistance results in an undesirable decrease in receiver output sensitivity, which adversely affects the sound quality of the earphone. The embodiments disclosed herein are aimed at overcoming the practical implementations of the use of inductors in conjunction with low frequency drivers as discussed above; however, this does not preclude inductors being implemented in conjunction with any of the embodiments disclosed herein.
Undesired higher frequency sound output from a low frequency driver can be filtered by increasing the sound passage length from the driver output to the output of the earphone. Acoustic inertance, which is the impeding effect of inertia on the transmission of sound in a duct of small cross-sectional area or the mass loading of air on the transmission of sound in a duct, can be calculated by the following equation, where ρ0 is the density of air and L is the length of the tube in meters, A is the cross-sectional area of the tube in meters-squared, and ω is the angular frequency of the sound wave in radians:
      Z    A    =                              ρ          0                ⁢        L            A        ⁢    jω  (in units of kg/m4).
As illustrated by the above equation, the acoustic impedance of the tube is directly proportional to both the length of the tube and the frequency of the excitation signal, and inversely proportional to the cross-sectional area of the tube. This acoustic mass element presents a reactive (i.e. energy absorbing) load to the acoustic pressure source, and as such, is analogous to an inductive element that presents a reactive load to a voltage source in the electrical domain. In the acoustic domain, this inertial load presents a linearly increasing impedance with frequency, thus serving as a first-order low-pass acoustic filter element. Therefore, an effective strategy to acoustically discriminate against higher frequency sound waves produced by the low frequency driver is to utilize a sufficiently large tube length in combination with a sufficiently small tube cross-sectional area. However, earphones worn in the ear canal are very small volumetrically, and for acoustic tubing commonly used in the art, it is very difficult to fit the required tube length within the earphone casing.
For example, short silicone tubes can be implemented to create a subtle low pass acoustic filter effect or tune a resonance peak to a target frequency, but a longer tube would need to be coiled or folded up in the small volume of an in-ear earphone, which may not be available to achieve the desirable performance. Although tubes may be used in conjunction with any of the embodiments disclosed herein, it proves difficult to use tubes to provide the appropriate length for the desired roll off of higher frequency sound waves with current earphone geometry, especially for multi-driver earphones.